Fail-safe railroad-highway grade crossing protection system

ABSTRACT

This disclosure relates to a fail-safe electronic system for protecting a railroad-highway grade crossing. The system includes a solid state transmitter for applying a constant current to the track and a solid-state receiver for obtaining a voltage signal from the track. A differentiating circuit senses any differential change between the track voltage and an opposing voltage in order to distinguish an approaching train from a receding train so as to appropriately control the warning apparatus. The integrity of the system is constantly monitored by modulating the track voltage signal so that the absence of the modulating signal activates the warning apparatus to ensure the highest degree of safety to pedestrians and motorists.

Darrow et al.

[ Dec. 25, 1973 FAIL-SAFE RAILROAD-HIGHWAY GRADE CROSSING PROTECTIONSYSTEM [75] lnventors: John O. G. Darrow, Murrysville;

Thomas C. Vaughn, Plum Borough,

both of Pa.

[73] Assignee: Westinghouse Air Brake Company,

Swissvale, Pa.

[22] Filed: Sept. 7, 1972 [21] Appl. No.: 286,872

[52] US. Cl. 246/128, 246/125 [51] Int. Cl B611 1/18 [58] Field ofSearch 248/125, 162, 128, 248/ 130 [56] References Cited UNITED STATESPATENTS 3,390,256 6/1968 Clanton et al. 248/125 X 3,610,920 10/1971Frielinghaus 246/128 Primary Examiner-Gerald M. Forlenza AssistantExaminerGeorge H. Libman Att0rney-H. A. Williamson et al.

[ 5 7] ABSTRACT This disclosure relates to a fail-safe electronic systemfor protecting a railroad-highway grade crossing. The system includes asolid state transmitter for applying a constant current to the track anda solid-state receiver for obtaining a voltage signal from the track. Adifferentiating circuit senses any differential change between the trackvoltage and an opposing voltage in order to distinguish an approachingtrain from a receding train so as to appropriately control the warningapparatus. The integrity of the system is constantly monitored bymodulating the track voltage signal so that the absence of themodulating signal activates the warning apparatus to ensure the highestdegree of safety to pedestrians and motorists.

14 Claims, 1 Drawing Figure Beafc'fder V and fsda d [fecewer Band PlusFalter" Law Pass Pder.

FAIL-SAFE RAILROAD-HIGHWAY GRADE CROSSING PROTECTION SYSTEM Thisinvention relates to a fail-safe railroad-highway grade crossingprotection arrangement, and more particularly to a vital type of warningsystem which effectively alerts and forewarns pedestrians and motoristsof an approaching train.

The, perils connected with the intersection of a roadway and railroadcrossing are universally known to the general public. It is commonpractice to provide a suitable warning to pedestrians and motorists byactivating an alarm circuit for indicating that a train or transitvehicle is approaching the grade crossing intersection. There areapproximately 232,000 railroadhighway grade crossings of which onlyabout 47,000 crossings have been equipped with protective warningdevices, such as, audible, visible, or barrier mechanisms to forewarnpedestrians and motorists of an approaching train or transit vehicle.Each year there are between 1,500 to 1,800 deaths and between 3,500 to4,000 injuries caused by accidents at crossings. In addition to thehuman anguish involved, there is an annual'economic loss well in excessof 300 million dollars due to the accidents at the crossings. Experiencehas shown that the accident rate at protected crossings is equal to andin some cases greater than that at unprotected crossings. ln protectedareas, a few accidents result from a system failure; however, mostmishaps are the result of human failings. For example, people becomeirritable and impatient at the crossings when the warning devices, suchas the flashing of the lights, the sounding of the gongs, or thelowering of the gate arms, are operated for a substantially long periodof time due to a slow approaching train or due to the train stopping inadvance of the crossing. Thus, many individuals fail to heed the warningafter an extended period of time and in numerous cases are clobbered bythe oncoming train in their vain attempt tomove across the intersection.In order to prevent or at least reduce the number of such needlessaccidents, it has been found to be advisable to deactivate the warningdevices when the train stops at some point in advance of the gradecrossing. It will be appreciated that a non-approaching train or vehiclepresents no danger to pedestrians or motorists at the crossing and,therefore, it is advantageous to allow the traffic to pass over thecrossing when the train is stopped in advance of a given point from thecrossing. Like a nonapproaching train, a receding train or vehicle,namely, one moving away from the grade crossing, presents no peril topedestrians or motorists attempting to cross the intersection. Thus, inaddition to deactivating the waming devices for a train stopped ahead ofthe grade crossing, it is highly desirable to effectively deactivate thewarning devices as soon as the last vehicle of a train passes a givensafe point beyond the grade crossing. In addition, any proposedrailroad-highway grade crossing monitoring system should effectively andefficiently control the warning devices in a manner that would providethe highest degree of safety yet would result in the lowest degree ofinconvenience and aggravation to the general public. Hence, theDepartment of Transportation has requested that grade crossingprotection be given the highest priority by all the parties involved andthat the industrial and governmental groups pursue the matter in allhaste. Further, the monitoring apparatus must operate in a vital orfail-safe manner in order to prevent costly damage to equipment as wellas to avert serious injury and possible death to individuals due tounsafe failures.

Hence,vit is an object of this invention to provide a new and improvedvehicle motion monitoring system for use in railroad-highway gradecrossings.

A further object of this invention is to provide a unique fail-safeelectronic system for monitoring the motion of a vehicle and foractuating warning devices when a vehicle approaches a grade crossing.

Another object of this invention is to provide a vital type of motionmonitoring arrangement which activates grade crossing warning equipmentwhen a vehicle is approaching the crossing and deactivates the gradecrossing warning equipment when the vehicle stops in advance of thegrade crossing or when the vehicle recedes from the grade crossing.

Still a further object of this invention is to provide a fail-safemonitoring arrangement for a railroadhighway grade crossing protectionsystem which initiates warning devices when a moving vehicle approachesin a given distance from the grade crossing.

Still another object of this invention is to provide an improvedrailroad-highway grade crossing warning system which forewarnspedestrains and motorists of an oncoming train by sensing a change inthe electrical impedance characteristic of the railroad track.

A still further object of this invention is to provide a uniqueautomatic crossing protection arrangement which alleviates undue delaysto the general public by only actuating the warning devices at a gradecrossing when a vehicle moves toward the crossing.

Yet another object of this invention is to provide a novelrailroad-highway gradecrossing protection arrangement which does notnormally perturb or irritate pedestrians and motorists in that theflashing lights, sounding bells, or the barrier gates are not activatedexcept when a train is moving toward the grade crossing.

Yet a further object of my invention is to provide a fail-safe gradecrossing warningarrangement for sensing the motion of railway vehicles.

In addition, it is a further object of this invention to provide a vitalrailroad-highway grade crossing protec-. tion system which is reliablein operation, durable in use, and efficient in service.

In accordance with the present invention, the failsafe railroad-highwaygrade crossing protection system includes a transmitter and a motionreceiver connected to the railroad track. The transmitter includes afailsafe constant amplitude signal generator having a shunt regulatorwhich supplies a substantially constant d.c. operating potential to afree-running transistor oscillator and preamplifier circuit. The ac.output of the os cillator preamplifier circuit is applied to a poweramplifier composed of a plurality of cascaded transistor amplifyingstages which provides a constant current output signal. The plurality ofstages ensure that sufficient amplification or gain is present forefficiently driving the load. The power amplifier is designed to have anexceptionally high negative feedback signal or large degeneration sothat the output of the amplifier is effectively a constant currentsources. One output terminal of the power amplifier is coupled to onerail of the track through a series resonant L-C network, the L parameterof which is formed by a primary winding of a voltage buckingtransformer, while the other output terminal of the power amplifier iscoupled to the other rail of the track through a low impedance windingof a suitable modulating transformer. Thus, it will be appreciated thatthe voltage that appears across the track will also remain substantiallyconstant when no railway vehicle is within the detection region of thegrade crossing protection system. The input terminals of the motionreceiver are coupled to the track rails by separate inductors, namely,the secondary windings of the voltage bucking and the modulationtransformer, respectively. The primary winding of the modulationtransformer is connected to an amplitude modulating circuit whichincludes a suitable type of oscillator, such as, a conventional lowfrequency m'ultivibrator so that the integrity of the system may beconstantly monitored against component and circuit failures which couldresult in an unsafe condition. The primary winding of the buckingtransformer supplies a large fixed voltage across the secondary windingwhich is effectively connected in series with the voltage developedacross the track rails. Thus, the algebraic sum of the track voltage,the bucking voltage and the modulating voltage is applied to the motionreceiver. The motion receiver includes a band-pass filter which passesthe desired frequency signals and substantially reduces or eliminatesspurious noise and other undesirable frequency signals. The output fromthe motion receiver isapplied to a multistage transistor amplifier, andthen the a.c. voltage is stepped up and rectified into a d.c. voltage.The d.c. voltage in turn is applied to a differentiating circuit whichproduces an output signal which is substantially in proportion to therate of change of the input voltage. The differentiator is coupled tothe input of a fail-safe oscillator which is normally powered byoperating voltage supplied from the shunt regulator of the tracktransmitter. The differentiator output is arranged to buck or oppose thenormal operating power of the oscillator so that the oscillations willcease when a train approaches the grade crossing, as will be describedin detail hereinafter. However, when no train is approaching the gradecrossing, the oscillator is powered by the voltage of the shuntregulator and produces a.c. oscillations which are supplied to asubsequent circuit. That is, the output of the fail-safe oscillator iscoupled to the a.c. input terminal of a two-input fail-safe AND gate. Inorder to provide the highest degree of safety to'the public, it ispreferable to supplement the motion monitor with a positive protectiontype of track circuit in the vicinity 'of the grade crossing. In thepresent case, a positive crossing protection area is achieved byemploying an island track circuit which is coupled to the. track railsat a given safe distance from the highway crossing. The island trackcircuit may consist of an AFO transmitter which is located at a selectdistance on one side of the highway crossing. An island receiver issituated at a suitable location on the other side of the highwaycrossing. The island track receiver includes a band-pass filter whicheliminates noise and any other extraneous signals. AF ter amplification,rectification and filtration, the received island signal is employed topower the d.c. input terminal of the two-input fail-safe AND logic gate.Thus, the lack of a non-approaching vehicle and the absence of a vehiclewithin the positive protection island track area cause both the a.c. andthe d.c. voltages to be applied to the input of the AND logic gate sothat an a.c. output signal is available at the output terminal of theAND logic gate. The output of the AND gate is connected to the input ofa modulation detector, a subsequent AND gate, and a wide-band amplifier.The output of the wide-band amplifier is rectified and filtered andprovides d.c. operating potential for the d.c. input terminal of thesubsequent AND gate. It will be seen that the d.c. operating potentialwill be present only when the modulation is detected or when the outputof the fail-safe oscillator is above a predetermined level. The presenceof both the d.c. operating potential from the wide-band amplifier andthe a.c. output from the preceding AND logic gate causes an a.c. signalto be passed by the second or subsequent AN D" gate. Afteramplification, the a.c. signal of the second AND gate is rectified andis employed to energize a polar sensitive type of relay which normallyopens a back contact and interrupts the warning equipment. Thus, thewarning equipment will remain deactivated so long as the polar relaycontinues to he energized. As mentioned above, when an approaching trainenters the'detection zone, the polar relay will become deenerzied sothat its back contact will become closed. The closing of the backcontact will cause the warning equipment to be activated, therebyalerting pedestrians and motorists of the impending danger.

The above, as well as other feature and objects of the present inventionwill be understood by reference to the following detailed descriptionwhen considered with the drawing in which the single FIGURE represents asimplified functional block diagram in the preferred embodiment of thepresent invention.

Referring now to the single FIGURE of the drawing,

there is illustrated a simplified functional block diagram of afail-safe railroad-highway grade crossing protection system of a typewhich preferably constitutes the present invention.

As mentioned above, a highway grade crossing protection system isactuated when a moving train enters the motion detector zone. Normally,the train detection zone is between 1,500 and 2,500 feet in advance ofthe crossing which is generally a safe distance from the highwaycrossing even when a train is traveling at its maximum speed. Undercertain circumstances, it is desirable to deactivate the highwaycrossing protection devices in cases where the train stops before itenters the positive protection area of the crossing in order thatvehicular traffic and pedestrians will not be needlessly inconveniencedby the stopped train. That is, in cases where a railway train stopsbefore a preselected minimum point in advance of the crossing, thewarning devices should be quickly deactivated so that the awaitingpublic may go over the crossing without undue delay to themselves.However, if the vehicle or train is restarted and again approaches thecrossing, it is absolutely necessary to promptly reactivate the warningdevices in order to protect persons attempting to go over the crossing.However, if, upon restart, the vehicle or train recedes or moves awayfrom the crossing, it is desirable to forego activating the warningdevices in that the receding train presents nodanger to peopleattempting to cross the grade crossing. in order to achieve a highdegree of effectiveness, the warning devices should be extinguished ordeactivated as soon as the train clears the crossing in order to readilypermit motorists and pedestrians to pass over the trackway. The warningsystem must operate in a vital manner in that no circuit or componentfailure should be capable of erroneously deactivating the warningdevices when an oncoming vehicle is approaching the crossing. Thus, eachand every component and portion of the system must be analyzed in afail-safe manner so that each and every precautionary measure may betaken to avert an unsafe condition.

Hence, the presently described invention is a fail-safe railroad-highwaygrade crossing protection system which includes a track transmitterhaving an oscillatorprearnplifier circuit and a power output amplifier1 1. The circuit 10 may take the form of a fail-safe constant amplitudesignal generator, as shown and described in my copending application forletters Patent of the United States, Ser. No. 108,264, filed Jan. 21,197 l for A Fail-Safe Constant Amplitude Signal Generator, which isassigned to the assignee of the present application. Theoscillator-preampifier or fail-safe constant amplitude signal generator10 includes a shunt regulator for supplying d.c. operating potential toa free-running transistor or oscillator as well as for controlling thequality factor of the resonant tuned circuit and, in turn, theoscillator so that an ac. output signal is capable of being producedwith a known amplitude which cannot be changed by a critical circuit orcomponent failure. The oscillator includes a transistor amplifying stagehaving a tickler coil for providing regenerative feedback and having aresonant circuit for determing the frequency of oscillation. The shuntregulator includes a pair of series connected resistors one of which isconnected to one terminal of a d.c. supply source and the other terminalof which is connected to the cathode of a Zener diode. The anode of theZener diode is connected to the other terminal of the d.c. source."Thus, constant a.c. amplitude signals are derived from the output of theamplifying transistor when and only when no critical circuit orcomponent failure is present. Now the preamplifier of circuit 10includes a series of transistor amplifier stages which are conventionaland well known in the art except for the fact that the input to thepreamplifier is safely filtered by a fourterminal capacitor. The gain ofthe preamplifier is extremely stable and independent of power supplychanges due to a relatively high amount of degenerated feedback. Thepower amplifier circuit 11 includes a plurality of cascaded transistoramplifying stages, the output stage of which may take the form of aclass B push-pull amplifier in order to produce high power output andefficiency. Further, since the supply voltage to the power amplifiercircuit 11 is also supplied by the shunt regulator of the oseilaltorcircuit, the output of the power amplifier will remain constant as longas the oscillator is working at all. Like the oscillatorpreamplifier,the power amplifier uses emitter degeneration to make it independent ofpower supply, and thus it supplies a constant current to the rails. Theindependence of power supply ensures that pulsations or ripples possiblepresent in the power source cannot result in a false amplitudemodulation of the transmitter output which could invalidate the systemcheck performed by the internal modulator. One output terminal of thepower amplifier 11 is coupled to one of the rails R1 of the track TR viaa series tuned resonant circuit made up of capacitor C and a primarywinding P1 of a voltage bucking transformer T1. The other outputterminal of the power amplifier 11 is coupled to rail R2 of the trackwayTR via a low impedance secondary winding S2 of a modulation transformerT2. The series tuned resonant circuit including capacitor C and inductorwinding P1 isolates the circuit from other a.c. frequency signals aswell as any d.c. signals which may be present on rails R1 and R2..Thus,the constant current is impressed upon the track rails, and assuming anormal 3 ohm ballast resistance, a train detection distance ofapproximately 2,500 feet is obtained with a 200 Hz signal, and adetection distance of approximately 1,500 feet is achieved with a 600 Hzsignal. Thus, a train entering the detection zone will vary the railimpedance so that a resulting voltage change will occur due to theshunting effect of the train wheels and axle.

As shown, the track voltage is constantly monitored by a motion receiver13. The motion receiver 13 is connected to the rail R1 through thesecondary winding S1 of the voltage bucking transformer T1 and isconnected to rail R2 through secondary winding S2 of the modulationtranformer T2. It has been found that the directional movement of thetrain must be sensed by subtracting the track voltage from a given fixedvoltage so that the voltage input to the differentiating eapactor isdecreased as a train recedes. This ensures that normally an outputvoltage from the differentiator should always be opposite in polarity tothe voltage polarity resulting when possible leakage occurs in thedifferentiating capacitor. Thus, transformer T2 is employed to provide alarge fixed voltage in series opposing relationship with the receivedtrack voltage. That is, primary winding Pl induces a large constantvoltage which is in opposition to the track voltage developed across thetrack TR so that an approaching train may be distinguished from areceding train, as will be more readily described hereinafter. It hasbeen found that in order to attain fail-safe operation at the gradecrossing, it is also necessary to provide some means of checking thesystem against failures. This type of vitalness is accomplished by amodulating scheme which induces a low frequency signal upon thesubstantially high frequency track voltage. As shown, a low frequencymultivibrator 14 is connected to primary winding P2 of modulatingtransformer T2. Thus, a modulating signal is impressed upon the trackvoltage and is also. fed to the motion receiver 13. The motion receiver13 includes a band-pass filter section which is tuned to the appropriatefrequency of the track voltage. The receiver also includes a surge diodenetwork which protects the circuit against high voltage transients, suchas, the spikes and transitory voltages which are produced by lightningand the like.

The output from the motion receiver 13 is fed to the input of amplifier15 which preferably includes a plurality of cascaded transistoramplifier stages. The output of amplifier 15 is transformer-coupled to arectifier and low pass filter 16. The filter 16 includes a voltagetransformer which steps up and increases the level of the ac. signal andprovides d.c. isolation. The rectification is accomplished by a voltagedoubler network which again increases the level of the receiver-voltagesignal. It will be understood that the rectified voltage'is negativewith respect to the supply voltage so that leakage through thedifferentiating capacitor would ensure the turn off of oscillator 18.The operation of the low pass filter is accomplished by a pair offour-terminal capacitorswhich ensure that a loss of a conductive lead orthe opening of the capacitor will not result in the passing of otherspurious frequency signals. The d.c. signals along with the lowfrequency modulating signal are fed to the input of a differentiatingcircuit 17. The differentiating circuit 17 includes an RC circuit forproducing an output which is substantially proportional to the'rate ofchange of the applied input signal. The capacitance value ofdifferentiator 17 is such that it readily allows passage of the lowfrequency modulating signal. As shown, the output of the differentiator17 is fed tothe input of an oscillator 18. it will be noted that theouptut of the differentiator 17 is in fact fed to a junction point .1which effectively is the power supply source terminal for the oscillator18. That is, the operatingpotential for the oscillator 18 is suppliedfrom a suitable supply source +V through a substantially larg resistanceR so that a substantially constant current is normally applied to theoscillator 18. The terminal +V, in fact, is tied to the shunt regulatorof the oscillatorpreamplifier circuit so that a stable constant currentsupply is available for oscillator 18. The output of the oscillator 18is supplied to the input of a first fail-safe AND" gate 19.

It will be appreciated that in order to provide the highest degree ofsafety to pedestrians and motorists using the railroad-highway gradecrossing, it is necessary to provide a positive protection area orsection on either side of the crossing. In the instant case, thepositive protection area is provided by an audio frequency overlay (APO)island track circuit arrangement. However, it is understood that othertypes of track circuits, both a.c. and d.c., may also be employed fordetecting when a vehicle or train is within bounds of the positivedetection area. The AFO track circuit includes an island transmitter.located on one side of the crossing, namely, on the right-hand side ofthe highway which is the safe minimum distance from the near edge of thehighway. The island track circuit also includes an island receiver 21which is connected to the opposite side of the highway, namely, on theleft-hand side as viewed in the drawing. Thus, the island track circuitis connected to the rails at a point which is the minimum distance fromthe crossing to alert motorists and pedestrians that a train is withinthe positive detection zone. Under certain circumstances it may bepossible to dispense with the island transmitter 20 and have the highwaycrossing HC located in the position. as shown in phantom in the drawing.Under such a condition, the island receiver 21 would obviously be tunedto the frequency of the oscillation signals produced by oscillatorcircuit 10.

It will be appreciated that the island receiver 21 preferably includes aband-pass filter network which eliminates various noise and otherextraneous'signals which may appear on the rails. The filtered outputvoltage of the island receiver 21 is applied to the input of amultistage transistor amplifier 22. The amplified AFO signals, in turn,are'applied to the input of a rectifier-filter network 23. The rectifierand filter network 23 includes an initial voltage doubling circuit and asubsequent four-terminal capacitor for rectifying and filtering the APOinput signals. The filtered d.c. output of the rectifier-filter circuit23 is applied to the dc. input of the first fail-safe AND" gate 19. Thefail-safe "AND" logic circuit 19 is preferably of the type generallydescribed and disclosed in letters Patent of the United States No.3,430,066, issued Feb. 25, 1969, to Donald B. Marsh and WalterW/Sanville, for a Fail- Safe AND Logic Circuit, which is assigned to theassignee of the present application. The AND" gate 19 includes an activenetwork in the form of an a.c. transistor amplifier which produces. alogical assertion,

namely, an a.c. output, during the presence of a pair of input signalsand which produces a logical negation, namely, no output, during theabsence of either or both of the input signals. For example, during thepresence of both the a.c. and the dc. input, the logic circuit functionsas a signal passing gate so that a.c. signals are readily available atthe amplifier output terminals. Alternatively, during the absence ofeither or both of the a.c. and the do input signals the logic circuit 19functions as a signal blocking gate so that no output signal isavailable at the output terminals. Further, the AND gate 19 operates ina fail-safe manner in that any critical componentor circuit failure willnot erro neously produce an a.c. output signal. Thus, an a.c. outputsignal is available from the first AND" gate 19 only when bothmodulation from the differentiator 17 and a signal from the islandreceiver are present.

As shown, the output from the first 'AND" gate logic circuit 19 isconnected to the input of a modulation detector 24, a ssecond orsubsequent fail-safe. AND logic gating circuit 25, and a modulationamplifier 26. The modulation detector 24 detects the presence or absenceof the modulating signal produced by the low frequency multivibrator 14and thereby provides a selfchecking effect on the electrical conditionand behavior of the oscillator 18 and on the integrity of thedifferentiator 17. This constant checking or examination procedure isnecessary in order to ensure that the capacitor of the differentiator 17has not become opencircuited whereby the oscillator 18 would continuallyoscillate irrespective of whether or not a train or vehicle wasapproaching or was within the detection zone of the highway crossing. Asshown, the output of the modulation detector 24 is coupled to the inputof the amplifier 26. Thus, since the failure, namely, opening of thecapacitor of differentiator 17, would result in the disappearance of thelow frequency modulating signal, the modulation detector 24 would beunable to provide an input to the amplifier 26, and therefore a circuitfailure would be readily detected; it will be appreciated that thecircuit failure immediately causes the activation of the warningapparatus so that traffic is obstructed until a maintainer or otherresponsible person can correct the situation. Under certain conditions arapidly receding train tends to cause the oscillator 18 to obliteratethe low frequency modulating signals. However, as shown, the amplifier26 is also indirectly coupled to the oscillator 18 via AND gate 19 sothat whenthe output signal of the oscillator is sufficiently orrelatively high, due to a rapidly receding train, the amplifier 26 willbe driven by this relatively high oscillator signal in that themodulating signal falls ofi and tends to effectively disappear due tothe rapidly receding train.

As mentioned above, the first fail-safe AND" gate 19 provides an a.c.input to the second fail-safe AND gate 25. it will be appreciated thatthe second fail-safe fAND gate may also be of the type shown anddisclosed in the above-mentioned US. Pat. No. 3,430,066. As shown, thedc. input of the fail-safe AND gate 25 is derived from a rectifier andfilter network 27 which is similar to circuits 16 and 23 and is composedof a voltage doubler network and a filtering circuit. The input to therectifier and filter circuit 27 is derived from the amplifier 26. Asmentioned, the output from the rectifier and filter circuit 27 iscoupled to the dc. input of the fail-safe AND" gate 25. Thus,

the d.c. input to gate will exist only when modulation is detected bymodulation detector 24 or when the oscillator level is sufficiently highdue to a receding train. The a.c. input to gate 25, which must alsoexist for producing an a.c. output, will only be present when the islandcircuit is not occupied or when a.c. oscillations are not produced byoscillator 18 due to an approaching train. All of. these circuitfunctions are performed in such a manner that no failure can result in aless restrictive condition with regard to the main function of detectingan approaching train. The output of the fail-safe AND gate 25 is appliedto a multistage solid-stage amplifier 28. The amplifier 28 includes aplurality of transistor stages which have sufficient gain to power avital type of electromagnetic relay 29. In the present instance, theoutput of the power amplifier 28 is rectified by diode which provides ad.c. power to the electromagnetic relay 29. As shown, the relay ismechanically coupled to a contact a which completes or interrupts thecircuit to suitable warning apparatus 31, which obviously may take theform of appropriate lights, bells, barrier gates, or any combinationthereof.

Turning now to the operation of the described railraod-highway gradecrossing warning system, it will be initially assumed that either norailway vehicle is ap proaching the highway crossing or, at least, thatno railway vehicle is within the detection area of the system. It willbe appreciated that under this condition the protective devicescontrolled by warning apparatus 31 should be deactivated if no circuitor component failure is present and the system is operating properly. Aspreviously mentioned, the track transmitter continuously supplies aconstant current to the track TR and, in the absence of an approachingvehicle, the voltage drop across the rails remains substantiallyconstant. The constant current flowing through the primary winding P1 oftransformer T1 also induces a substantially large constant voltage intothe secondary winding S1 of transformer T1. It will be recalled that themotion receiver 13 is connected to the track rails R1 and R2 through thesecondary windings S1 and S2, respectively. Thus, in addition toreceiving the track voltage signal, the motion receiver 13 is suppliedwith the bucking voltage developed across secondary winding S1 and alsothe modulating signal induced into the secondary winding S2 by themultivibrator 14. Thus, the algebraic sum of the track voltage, thebucking voltage, and the modulating voltage is applied to the motionreceiver 13. As shown, the bucking voltage is induced into a relativelysmall secondary winding of the transformer T1 and any failure whicheliminates the bucking voltage causes the removal of input voltage tothe receiver 13. In addition, any detuning effect will greatly increasethe impedance of the series resonance circuit formed by the primarywinding P1 and capacitor C1 so that such an adverse condition willresult in the reduction of the amount of current that is applied to therails R1 and R2. It will be appreciated that the amount of modulation isproportional to the amount of current applied to the rails and,therefore, either of the abovementioned adverse conditions will reducethe modulaand rectified by the circuits 15 and 16, respectively.

The rectified output is applied to the differentiating circuit 17. Itwill be appreciated that the differentiating circuit 17 is responsive tothe rate of change of the incoming voltage. However, with the lack of anoncoming train, the difference between the bucking voltage and the trackvoltage remains unchanges so that no differential voltage appears acrossthe resistive capacitive elements of the differentiating circuit 17.Thus, the power supplied to the oscillator 18 during the absence of atrain within the detection zone is in effect +V so that the current 1flowing to junction .1 is equal to V/R. It will be appreciated that thelow pass filter of circuit 16 and the capacitor of differentiator 17freely allow passage of the low frequency modulating signal/The supplycurrent allows the oscillator 18 to go into oscillation and provide ana.c. signal to the a.c. input termi nal of the fail-safe AND gate 19.Further, the absence of a train within the positive protection zone ofthe highway crossing covered by the island track circuit allows the AFCsignals emanating from transmitter 20 to be received by the islandreceiver 21. The signals of the island receiver 21 are amplified,rectified, and filtered by the circuits 22 and 23, respectively, and ad.c. output from the rectifier is applied to the d.c. input terminal ofthe fail-safe AND gate 19. Thus, the a.c. oscillator frequency signalsare passed by the first failsafe AND gate and are applied to the a.c.input terminal of the second fail-safe AND" gate 25. The output signalof the oscillator 18, which is passed by the fail-safe AND gate, isaccompanied by the low frequency modulated signal which is detected bythe modulation detector 24. The modulation detector 24 provides amodulation frequency input signal to the amplifier 26, the output ofwhich, in turn, is applied to the input of the rectifier and filter 27.It will be appreciated that an exceedingly rapidly receding train withinthe detection zone results in the overloading of the modulation detector24 and, accordingly, the increased amplitude of the oscillator outputactivates the other input to the amplifier 26. Thus, the d.c. signalfrom rectifierfilter 27 is applied'to the d.c. input terminal of thesecond failsafe AND gate 25. The presence of the a.c. input signal, aswell as the d.c. input signal to gate 25, results in the passage of thea.c. signal which, in turn, is applied to the input of amplifier 28. Theamplified output signal of the amplifier 28 is, in turn, rectified byhalf-wave diode rectifier 30. The rectifier signal energizes theelectromagnetic relay 29, so that the front contact a is held open. Theopen contact interrupts the warning circuit so that the warningapparatus 31 is de activated, and traffic is allowed to pass freely overthe, highway crossing HC.

Let us now assume that a train enters the detection zone which, asmentioned above, may be between upon the frequency of thetransmittersignal. Upon entering the detection zone, the approaching train beginschanging the rail impedance so that the voltage developed across therails R1 and R2 varies in accordance to the differentiating circuitwhich produces an output in proportion to the rate of change of the die.input voltage. The voltage fed to the motion receiver and hence thevoltage applied to the differentiator is arranged to increase as a trainapproaches so that any differentiator capacitor leakage will appear asan approaching train which would be a safe failure. It can be seen thatcontinuity of the difi'erentiator 17 is checked by its ability to passthe. modulating signal. Thus, the output current produced by thedifferentiator 17 is adjusted to be substantially equal or greater thanand opposite to the operating current supplied to the junction J by thevoltage source +V. Hence, the sum of the currents at junction]is-effectively zero or negative so that the oscillator 18 is effectivelywithout power of the proper polarity. Under this condition, theoscillator 18 reverts to a nonoscillating condition so that oscillatorfrequency signals are not available at the a.c. input terminal of ANDgate 19. Further, the absence of a.c. oscillator frequency signals atthe ac. input terminal of the fail safe AND gate 19 obviously results inno a.c. signal at its output terminal. Therefore, there is no a.c.signal available at the output on the second fail-safe AND gate and, inturn, there is no voltage available forenergizing the electromagneticrelay 29. Thus, the relay 29 becomes deenergized and the back contact areleases and closes so that the warning apparatus 31 immediately becomesenergized. The relay 29 will remain deenergized so long as the traincontinues to approach the highway crossing HC. If, for any reason, thetrain should stop in advance of the highway crossing and outside thebounds of the island track circuit, the

rate of change in voltage ceases so that the differentiator 17 will nolonger produce a current which opposes the current supplied from the +Vterminal of the normal supply source. Accordingly, the oscillator 18will immediately go into an oscillating condition upon the stopping of arailway vehicle which is in approach of the island track circuit, andthus the warning apparatus 31 will be prompty deactivated.

It will be appreciated that when the train comes within the bounds ofthe positive protection area, namely, within the island track circuit,the transmitter signals of island transmitter 20 are shunted by thewheels and axle of the train, and, therefore, the island receiverreceives no AFO input signal. Thus, there is no dc voltage applied tothe d.c. terminal of the first failsafe ANDT gate 19. Hence, absence ofa dc. input causes the AND" gate 19 to assume a block mode of 7operation, namely, a.c. oscillations-will not appear on the output ofgate 19.

However, upon restart, an approaching train will again cause thedifl'erentiator 17 to supply a current to junction J which is inopposition to the normal supply current which, therefore, will cause theoscillator 18 to stop oscillating again. However, on restart, a recedingtrain causes the differentiator current to enhance the current of thenormal supply source so that the oscillator 18 will continue tooscillate and, therfore, there is no change in operation and the warningapparatus will remain deactivated. When the the differentiator currentbecomes large enough to overload the modulation detector 24, the outputof oscillator 18 will be sufficiently high enough to directly activateamplifier 26. That is, the warning apparatus will remain deenergizedwhen a stopped train restarts and begins to recede from the highwaycrossing in the direction it came from.

Similarly, when the last vehicle of the train exits the island trackcircuit, a receding signal will be received by the motion receiver 13which, in turn, causes the difierentiator 17 to supply a current to thejunction J which enhances the normal supply current. Thus, a.c.oscillations are produced by the oscillator 18 as soon as the trainpasses the limits of the island track circuit. Hence, since'a recedingtrain does not endanger motorists and pedestrians, it is thereforedesirable to deactivate the warning apparatus 31, such as lights, bells,or barrier, as soon as possible in order not to inconvenience thegeneral public.

. As previously mentioned, the modulating signal provides a fail-safechecking arrangement for ensuring that a circuit or component failurewill not result in not providing an adequate warning to the public. Theuse of a bucking voltage allows a more effective and efficient system inthat it is readily capable of determing the difference between anapproaching and a receding train.

It will be appreciated that various changes, modifications andalterations may be made by persons skilled in the art without departingfrom the spirit and scope of the present invention. Thus, it will beunderstood that various modifications may be made in the presentlydescribed invention and, therefore, it is realized that all changes,equivalents, and mutations within the spirit and scope of the presentinvention are herein meant to be covered by the appended claims.

Having thus described my invention, what i claim is:

l. A fail-safe electronic system for protecting a railroad-highway gradecrossing comprising, transmitter means for applying a constant currentsignal to the track, receiver means for receiving a voltage signal fromthe track, means coupled to said receiver means for producing a voltagepotential which is in bucking relationship with said track voltagesignal, means coupled to said receiver means for modulating said trackvoltage signal, differentiator means coupled to said receiving means forproducing an output signal substantially proportional to the rate ofchange of the input signal, oscillator means coupled to saiddifferentiator means, a track circuit coupled to the track andencompassing the grade crossing, and means coupled to said I oscillatorand said track circuit and responsive to the condition of saidoscillator and said track circuit for actuating a protective warningwhen a train is approaching thegrade crossing and is outside the limitsof said track circuit or when a train is within the limits of said trackcircuit or when a critical circuit or component failure occurs withinthe system.

2. A fail-safe electronic system as defined in claim 1, wherein atransformer provides said bucking voltage to said receiver means.

3. A failsafe electronic system as defined in claim 1, wherein saidmodulating means is transformer coupled to said receiver means.

4. A fail-safe electronic system asdel'ined in claim 1, wherein saiddifierentiator means is coupled to the power supply of said oscillatormeans so that the power supply voltage is opposed by voltage supplied bysaid differentiator means when a train is approaching the gradecrossing.

S. A fail-safe electronic system as defined in claim 1, wherein saiddiflerentiator means is coupled to the power supply of said oscillatormeans so that the power supply voltage is enhanced by voltage suppliedby said differentiator means when a train is receding from the gradecrossing.

6. A fail-safe electronic system as defined in claim 1, wherein saidresponsive means includes a first AND logic gate having two inputs andan output in which one input is connected to said oscillator means andthe other input is connected to the track circuit.

7. A fail-safe electronic system as defined in claim 6, wherein saidoutput of said first AND logic gate is connected to a modulationdetector for detecting the modulated signal of said modulating means, toa modulating amplifier and to a second AND logic circuit.

8. A fail-safe electronic circuit as defined in claim 6, wherein saidsecond AND logic gate requires an input signal to be applied from saidfirst AND logic gate and an input from said modulation amplifier inorder to produce an output.

9. A fail-safe electronic system as defined in claim 1, wherein saidtrack circuit is an island circuit having a transmitter means connectedto the track on one side of the grade crossing and a receiver meansconnected to the track on the other side of the grade crossing.

10. A fail-safe electronic system as defined in claim 6, wherein saidtrack circuit includes a receiver means the output of which is amplifiedand rectified and then is applied to the other input of said first ANDgate.

11. A fail-safe electronic system as defined in claim 1, wherein saidtransmitter means is connected to the track through a tuned resonantcircuit.

12. A fail-safe electronic system as defined in claim 11, wherein saidtuned resonant circuit includes the primary winding of a transformer.

13. A fail-safe electronic system as defined in claim 12, wherein saidtransformer includes a, secondary winding which is connected to saidreceiver means and into which is induced said bucking voltage potential.

14. A fail-safe electronic system as defined in claim 1, wherein saiddifierentiator means opposes the power supply of said oscillator meanswhen a train approaches the grade crossing so that said oscillator meansassumes a non-oscillatory condition and said differentiator meansenhances the power supply of said oscillator means when a train recedesfrom the grade crossing so that said oscillator means assumes anoscillatory condition.

1. A fail-safe electronic system for protecting a railroadhighway gradecrossing comprising, transmitter means for applying a constant currentsignal to the track, receiver means for receiving a voltage signal fromthe track, means coupled to said receiver means for producing a voltagepotential which is in bucking relationship with said track voltagesignal, means coupled to said receiver means for modulating said trackvoltage signal, differentiator means coupled to said receiving means forproducing an output signal substantially proportional to the rate ofchange of the input signal, oscillator means coupled to saiddifferentiator means, a track circuit coupled to the track andencompassing the grade crossing, and means coupled to said oscillatorand said track circuit and responsive to the condition of saidoscillator and said track circuit for actuating a protective warningwhen a train is approaching the grade crossing and is outside the limitsof said track circuit or when a traiN is within the limits of said trackcircuit or when a critical circuit or component failure occurs withinthe system.
 2. A fail-safe electronic system as defined in claim 1,wherein a transformer provides said bucking voltage to said receivermeans.
 3. A fail-safe electronic system as defined in claim 1, whereinsaid modulating means is transformer coupled to said receiver means. 4.A fail-safe electronic system as defined in claim 1, wherein saiddifferentiator means is coupled to the power supply of said oscillatormeans so that the power supply voltage is opposed by voltage supplied bysaid differentiator means when a train is approaching the gradecrossing.
 5. A fail-safe electronic system as defined in claim 1,wherein said differentiator means is coupled to the power supply of saidoscillator means so that the power supply voltage is enhanced by voltagesupplied by said differentiator means when a train is receding from thegrade crossing.
 6. A fail-safe electronic system as defined in claim 1,wherein said responsive means includes a first AND logic gate having twoinputs and an output in which one input is connected to said oscillatormeans and the other input is connected to the track circuit.
 7. Afail-safe electronic system as defined in claim 6, wherein said outputof said first AND logic gate is connected to a modulation detector fordetecting the modulated signal of said modulating means, to a modulatingamplifier and to a second AND logic circuit.
 8. A fail-safe electroniccircuit as defined in claim 6, wherein said second AND logic gaterequires an input signal to be applied from said first AND logic gateand an input from said modulation amplifier in order to produce anoutput.
 9. A fail-safe electronic system as defined in claim 1, whereinsaid track circuit is an island circuit having a transmitter meansconnected to the track on one side of the grade crossing and a receivermeans connected to the track on the other side of the grade crossing.10. A fail-safe electronic system as defined in claim 6, wherein saidtrack circuit includes a receiver means the output of which is amplifiedand rectified and then is applied to the other input of said first ANDgate.
 11. A fail-safe electronic system as defined in claim 1, whereinsaid transmitter means is connected to the track through a tunedresonant circuit.
 12. A fail-safe electronic system as defined in claim11, wherein said tuned resonant circuit includes the primary winding ofa transformer.
 13. A fail-safe electronic system as defined in claim 12,wherein said transformer includes a secondary winding which is connectedto said receiver means and into which is induced said bucking voltagepotential.
 14. A fail-safe electronic system as defined in claim 1,wherein said differentiator means opposes the power supply of saidoscillator means when a train approaches the grade crossing so that saidoscillator means assumes a non-oscillatory condition and saiddifferentiator means enhances the power supply of said oscillator meanswhen a train recedes from the grade crossing so that said oscillatormeans assumes an oscillatory condition.